Recently, an electronic paper apparatus using a technique, such as electrophoresis or electrochromic, thermal, dichroic particle rotation methods, and the like, has been actively developed as an image display capable of replacing a liquid crystal display.
Since such an electronic paper apparatus secures wider viewing angle, close to that of general printed matter, than liquid crystal displays, provides low power consumption, and is economical, it is anticipated that many next generation image displays will include such electronic paper.
The electronic paper apparatus is operated by two representative methods. Here, one is a method of operating the apparatus by filling a microcapsule with ink and the other is a method of operating the apparatus by filling a microcup with an ink. In particular, the microcup method has merits in that a preparation method is simple and it is possible to allow most processes to be performed in-line.
A microcup type electronic paper apparatus has a structure as shown in FIG. 1. In the structure, microcups 10 having a height of 15 μm to 30 μm and a length and width of 5 μm to 180 μm are arranged, and charged particles (typically, titanium dioxide) are dispersed in an organic solvent inside each of the cups 10.
The microcup type electronic paper apparatus includes ITO electrodes 30 disposed on upper and lower sides of the microcups 10, such that, when voltage is applied to the ITO electrodes 30, the charged particles dispersed in the organic solvent are arranged to realize black, white or other colors.
In the microcup type electronic paper apparatus, partition walls 15 are formed to prevent horizontal movement of the charged particles.
Such partition walls 15 serve to separate pixels and to maintain a space of the microcup. In addition, when the microcup type electronic paper apparatus is applied to a flexible display, the partition walls 15 must be kept bonded to the upper and lower ITO electrodes 30 without detachment even when the apparatus is bent, while serving as a supporter maintaining the space of the microcup.
Otherwise, when the electronic paper apparatus is bent, the partition walls 15 are frequently detached from the upper and lower ITO electrodes 30. In this case, since the charged particles placed in each pixel space move to other adjacent cups 10, image quality becomes unstable and the pixel is deteriorated, thereby causing increase in product failure.
Thus, the microcup type electronic paper apparatus includes a sealing material 20 disposed between the ITO electrode 30 and the microcups 10. The partition walls 15 and the ITO electrode 30 must be bonded to each other well via the sealing material 20 such that the solvent and the charged particles confined in the microcup 10 do not leak from the microcups 10.
FIGS. 2a to 2c are sectional views showing operation of bonding the ITO electrode 30 of the electronic paper apparatus to the partition walls 15.
First, FIG. 2a shows a method in which the organic solvent and the sealing material incompatible with the organic solvent are introduced into the microcups 10, followed by final curing when the sealing material gathers at upper sides of the microcups 10 due to phase separation between the sealing material and the solvent, thereby forming the sealing material 20.
FIG. 2b shows a method in which the sealing material 20 is coated onto upper surfaces of the partition walls 15, followed by bonding the ITO electrode 30 to the partition walls 15, and FIG. 2c shows a method in which the sealing material 20 is coated onto an overall surface of the ITO electrode 30, followed by bonding the ITO electrode 30 to the partition walls 15. That is, the sealing material 20 is coated onto the upper surfaces of the partition walls 15 or the entire surface of the upper ITO electrode 30 using a roller, and the upper ITO electrode 30 is then stacked on the partition walls 15, followed by curing the sealing material 20 through UV irradiation of the upper ITO electrode 30, thereby bonding the partition walls 15 to the upper ITO electrode 30.
However, to apply the sealing material 20 to the microcup type electronic paper apparatus, the sealing material 20 must be able to embed the partition walls 15 therein and exhibit exact interfacial separation through phase separation due to poor compatibility with ink. Thus, the method illustrated in FIG. 2a has a problem of low reliability.
In addition, the methods illustrated in FIGS. 2b to 2c have problems in that the liquid sealing material dissolves in the organic solvent inside the microcups before curing to widely spread, causing blurring of the pixel space, or in that the charged particles stick to the sealing material due to tack at room temperature to influence charge states, causing unstable operation of the apparatus.
FIG. 3 shows a typical sealing film 100. In the typical sealing film 100, a sealing layer 1 is formed as a single layer on a release layer 3. For the sealing film 100, a composition exhibiting high peel strength is generally used to improve sealing performance between the sealing layer 1 and the partition walls of the microcups. In this case, tack of the sealing layer 1 causes a particle freezing phenomenon wherein the charged particles inside the microcup stick thereto, thereby deteriorating optical properties. In addition, when the sealing layer 1 is formed of a tack-free composition at room temperature to prevent particle freezing, the sealing layer does not have improved sealing performance, although optical properties can be satisfied, thereby making it difficult to satisfy long-term durability.